Aspects of the present invention are directed to lot process order modification to improve detection of manufacturing effects.
The manufacture of most products, such as wafers containing semiconductor devices, requires a number of discrete processing steps to create the product. For wafers, a number of discrete steps are needed to produce a packaged semiconductor circuit device from raw semiconductor material. The starting substrate is usually a slice of single crystal silicon referred to as a wafer. Circuits of a particular type are fabricated together in batches of wafers called “lots” or “runs”. The fabrication process creates regular arrays of a circuit on the wafers of a lot. During processing, the individual wafers in a lot may go through individual processing steps one at a time or as a batch. There may be hundreds or thousand of these processing steps before completion. At the completion of wafer processing, the wafers are tested to determine circuit functionality. Later, the wafers are sliced, the functioning products are packaged and further testing occurs.
In such lot based, multi-step processing, an effect from a single process step (PDID) in the route may degrade the processing performance with a distinct temporal signature. For example, the performance may shift from one performance level to another due to a piece of equipment malfunctioning. If the lot sequence is unchanged throughout the route, however, a graphical analysis of the time series of the performance vs. the lot sequence at each process step will look identical. This will make it impossible to use charts or time series analysis to distinguish between all the possible process steps as possible sources of the temporal signature.
The extent to which the lot sequence is not identical at each process step provides sensitivity to a time series analysis of the performance vs. lot sequence data on a process step basis. Currently, in many lot based, multi-step processes, lot order changes do occur as a result of lot priorities, holds, tools faults and/or other similar issues. However, the effects of these lot order changes are relatively minor and cannot be used to significantly alter the appearance of the graphical analysis of the time series of the performance vs. the lot sequence.